This invention relates to a control apparatus for the output voltage or current of a solar cell or array of solar cells, and particularly relates to those in which variable light transmissive devices, such as liquid crystal devices, are used as control regulators.
Solar cells and solar arrays are in widespread use for generation of electrical power, especially in remote or inaccessible locations. In particular, solar cell arrays are used to provide electrical power on spacecraft. FIG. 1 illustrates, in highly simplified form, a prior art spacecraft 10 including a body 12 and a pair of solar panels or solar cell arrays 14a, 14b mounted on corresponding booms 16a, 1b. Booms 16a and 16b may be arranged on bearings, such as bearing 18b, to allow for rotation of the booms for positioning the solar cell arrays.
FIG. 2 is a simplified block diagram of a prior art arrangement for controlling the output voltage of the solar arrays of a spacecraft. In FIG. 2, the solar panels 14a and 14b of FIG. 1 are illustrated together as a single block designated 14. When sunlight falls on solar panel 14 of FIG. 2, an electrical voltage is generated between terminals 20 and 22, which voltage can be used to energize a utilization apparatus, illustrated as a resistor 24. Electrical current flows under the impetus of the solar panel voltage from terminal 20, through a bus conductor 26 and slip rings 28 to load 24. Slip rings 28 are provided to allow a continuous electrical path notwithstanding the rotation of booms 16 relative to body 12 of the spacecraft of FIG. 1. The return path for current from load 24 to solar panel 14 is by way of another slip ring 28b and, as is well known to those skilled in the art, another conductor, illustrated in FIG. 2 as the common ground on the spacecraft.
In FIG. 2a, a resistor 30 and a zener or avalanche diode 32 form a voltage divider 33 which is coupled between bus 26 and ground, for generating reference voltage at node 34. A controller illustrated as a block 36 includes an amplifier (not illustrated) which compares a sample of the voltage from bus 26 with the reference voltage at node 34, and produces an error signal which is applied by way of a conductor 90 to the control input terminal of a shunt load, illustrated as a block 38. Shunt load 38 is coupled by a slip ring 28c across a portion of the solar array. Shunt load 38 responds to the error voltage by varying its conduction, to thereby interact with the current producing ability of solar panel 14 in a manner which reduces the voltage on bus 26 toward the value established by control block 36. Such schemes are notoriously well known and are but one form of a degenerative or negative feedback control system.
FIG. 2b illustrates a portion of solar panel 14. In FIG. 2b, solar panel 14 includes a plurality of arrayed solar cells 40a, 40b, 40c. . . , each of which has an upper surface 41 and a lower surface. Upper surface 41 includes a light input port or aperture, which is merely a light-sensitive portion of surface 41. When light, illustrated by a photon symbol 46, "enters" the aperture, a voltage is generated between upper surface 41 and the lower surface of each solar cell 40. Conductors (not illustrated) associated with the upper and lower surfaces of each solar cell provide ohmic contact between interconnection conductors 42 and the solar cells. Each conductor 42 of FIG. 2b connects the top surface of one solar cell to the bottom surface of the adjoining solar cell, thereby forming an electrical series connection by which the individual voltages produced by each cell can be added to produce a larger output voltage. Parallel connections of a number of such series strings may be made to increase the current output capability of the solar panel. Upper surface 41 of each solar cell 40 is protected by a coverglass 44, as for example upper surface 41 of solar cell 40b is protected by a coverglass 44b. The coverglass may itself have a conductive coating on its outer surface (the surface remote from the solar cell) to reduce arc discharges.
A disadvantage of the scheme of FIG. 2 lies in the thermal problems of spacecraft and the high power dissipation of shunt load 38 of FIG. 2 during operation. In a particular spacecraft under design, a solar array is used in which 90 solar cells are connected in series, each of which produces about 0.5 volts at maximum power. A sufficient number of cells are parallelled to produce an output current of 40 amperes (A), and shunt regulation is used in conjunction with 58 of the 90 series-connected cells to maintain a desired bus voltage of 28 volts. The 32 series cells which are not shunt regulated produce a maximum of 16 volts, whereupon the voltage across the 58 cells being regulated must be reduced from as much as 29 volts to 12 volts to meet the desired bus voltage. Assuming that the maximum current being shunted is 30 A, the total shunt power is 30 A multiplied by 12 volts, which equals 360 watts. In the above-mentioned spacecraft design, dissipation of this amount of heat requires that the shunt circuits be mounted on heat sink brackets associated with the solar array. The shunt elements and the associated brackets in this particular instance weigh approximately 22 lbs. It is desirable to reduce the high thermal dissipation in a shunt load, and to eliminate the weight associated with such a control scheme.